The present invention relates to foam-forming mixtures in which the dehydrohalogenation of a hydrohalocarbon blowing agent is substantially avoided or reduced, a process for the production of foams from such mixtures and to the rigid foams produced from these mixtures.
It is known that rigid, low density polyurethane, polyurethane urea and polyisocyanurate foams may be produced by reacting and foaming a mixture which includes an organic diisocyanate or polyisocyanate with an isocyanate reactive mixture in the presence of a volatile blowing agent which vaporizes at temperatures generated during the reaction.
In the commercial production of rigid polyurethane, polyurethane urea and polyisocyanurate foams, chlorofluorocarbons such as trichlorofluoromethane have been used as the blowing agent. However, these chlorofluorocarbons are believed to have an adverse effect upon the earth's ozone layer. Replacements for these chlorofluorocarbons have therefore been sought.
At present, the most promising chlorofluorocarbon replacements appear to be the hydrohalocarbons. Because these hydrohalocarbons are less stable in the atmosphere than chlorofluorocarbons over an extended period of time, it is expected that the hydrohalocarbons will cause less damage to the ozone layer. However, these hydrohalocarbons have a greater tendency to degrade under foam-forming conditions than their chlorofluorocarbon predecessors. The hydrohalocarbons undergo dehydrohalogenation to form halogenated alkenes. They may also undergo reduction reactions in which halogen atoms are replaced with hydrogen.
One solution to the chlorofluorocarbon degradation problem which was suggested by Hammel et al in a paper entitled "Decomposition of HCFC-123, HCFC-123a, and HCFC-141b in Polyurethane Premix and in Foam," was to wait to add the HCFC (i.e., hydrogen-containing chlorofluorocarbon) to the foam-forming mixture until just before use. This solution is not, however, practical in commercial foam production processes.
Means for stabilizing hydrohalocarbons under foam forming conditions have, therefore, been sought by those in the art. U.S. Pat. No. 5,137,929, for example, teaches that inclusion of certain types of stabilizers in a foam-forming mixture reduces the amount of decomposition of a hydrohalocarbon blowing agent during the foaming process. Among the materials taught to be useful as stabilizers are esters, organic acids, anhydrides, aminoacids, ammonium salts, bromoalkanes, bromoalcohols, bromoaromatic esters, chloroalcohols, nitroalkanes, nitroalcohols, triarylmethyl chlorides, triarylmethyl bromides, 3-sulfolene, zinc dialkyldithiophosphate, haloalkyl phosphate esters, carbon molecular sieves, powdered activated carbon, zeolite molecular sieves, sulfonate esters, and haloalkyl phosphate esters.
In a paper entitled "Minimization of HCFC-141b Decomposition in Rigid Polyisocyanurate Foams," Bodnar et al takes a different approach. Bodnar et al recommends that the catalyst employed in the foam-forming reaction be selected so that any compatibilizer in the polyol will not be able to solvate the cation of the catalyst and thereby render the anion of the catalyst more reactive.